Band pass amplifier



April Z, 193% R. A. BRADEN BAND PASS AMPLIFIER Filed Dec. 3, 1929 INVENTOR RENE A. BRAEN TTRNEY Patented Apr. 7, 1936 UNITED STATES BAND PASS AMPLIFIER Rene A. Braden, New

York, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application December 3, 1929, Serial No. 411,260

7 Claims.

My present invention relates to radio frequency amplifiers, and, more particularly to amplifiers adapted to pass a band of frequencies of substantially constant width.

In co-pending applications, Serial No. 278,105, filed May 16, 1928 for Tuned radio frequency amplifier and Serial No. 390,468, filed Sept. 5, 1929 for Band-pass amplifier, I have disclosed radio frequency amplifiers of the band pass type in which a constant band width characteristic is secured by varying the coupling between the primary and secondary circuits in such a manher that the co-eificient of coupling is proportional to the wave length, or inversely proportional to the wave frequency. In these aforesaid applications there were disclosed various means of novel construction for accomplishing the coupling variation.

By further experimentation I have discovered and devised additional methods for adjusting the coupling between tuned circuits employed in band pass amplifiers. Briefly, the basic principle embodied in my present discovery depends upon a well known phenomenon of electromagnetism. I have found that it is possible to control the coupling between two coupled circuits by utilizing the currents flowing through the coupling coils of the two circuits to generate an independent fiow of current in an additional circuit adjacent the said coupling coils, the magnetic field resulting from the flow of current in the additional circuit opposing the original magnetic field set up by the fiow of current through the coupling coils. It will, therefore, be immediately realized that the effective magnetic field surrounding the coupling coils may be controlled with facility by merely regulating the intensity of the additional or auxiliary opposing magnetic field.

Accordingly, it can be stated that one of the main objects of my present invention is to provide a method of, and means for, coupling tuned circuits employed in band pass amplifiers in such a manner that the strength of the coupling magnetic field between the tuned circuits may be controlled in a predetermined manner by an auxiliary magnetic field independent of and opposed to, but produced by, the said coupling magnetic field.

Another important object of the invention is to provide a radio frequency amplifier of the band pass type, which amplifier comprises a plurality of tuned circuits, each pair of tuned circuits being coupled by a magnetic field of constant strength, and means, independent of the tuned circuits, adapted to be disposed within the coupling magnetic field whereby an additional magnetic field is produced in opposition to the original coupling magnetic field, and further means adapted to vary the degree of opposition between the original and independent magnetic fields simultaneously with change in wave frequency.

Other objects of the invention are to improve generally the simplicity and efficiency of coupling arrangements between band pass amplifier tuned circuits, and to provide coupling adjusting devices which are economically produced, and reliable in operation.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing:

Fig. 1 diagrammatically shows a basic arrangement' of the present invention,

Fig. 2 is a diagrammatic representation of the arrangement in Fig. 1 applied to an amplifier circuit,

Fig. 3 shows diagrammatically a modification of the arrangement in Fig. 1,

Fig. 4 shows the arrangement of Fig. 3 applied to a receiving circuit.

Referring to the accompanying drawing in which like characters of reference in different figures indicate the same parts, there is disclosed in Fig. 1 a pair of coupled circuits, only the coupling portions of the circuits being shown for the sake of simplicity. One of the circuits includes an inductance coil l, in series with a coupling coil 2 while the other circuit includes an inductance coil 4, of similar characteristics as the coil l. The coil 4 is in series with a coupling coil 3, the coils 2 and 3 being wound together on an insulating tube (not shown) in such a manner that the turns of the coils 2 and 3 are interlinked as closely as possible.

This manner of interlinking the coils on an insulating tube is very well known to those skilled in the art, and need not be described with greater detail. While I have stated that the coils I and 4 are of similar characteristics, it is to be understood that they need not be necessarily so. A copper tube 5 fits closely over the intertwined coils 2 and 3, which are insulated so as not to make metallic contact with 5. The tube 5 is supported on sliding members (not shown) which permit movement of the tube parallel to its axis, so that the extent to which the coils 2, 3 are covered by the tube 5 can be varied.

It is to be understood that coils I and 2 constitute the primary circuit, and coils 3 and 4 the secondary circuit of a band pass amplifier. The primary circuit is coupled to the secondary circuit by the mutual inductance between the coils 2 and 3. Because of the close interlinking of the windings 2 and 3, the co-efiicient of coupling between these two coils is practically unity, and the mutual inductance is practically equal to the self-inductance of either coil. This last statement is made on the assumption that the coils are equal; which is the usual condition, though not a necessary one.

The metallic ring 5 may be composed of aluminum, silver, or any other low resistance metal, besides being composed of copper. It will be seen that when the ring or tube 5 is within the magnetic field set up around the interlinked coupling coils 2 and 3, the voltage induced in the member 5 causes current to flow around it. This current, in, accordance with a well-known phenomenon of electro-magnetism, induces a magnetic field in opposition to the original coupling field around coils 2 and 3.

The result is, therefore, that the effective coupling field around coils 2 and 3 is reduced in proportion to the proximity to the tube or ring 5. Those skilled in the art will readily understand that the reduction of the strength of the original magnetic field, is necessarily a reduction of coupling between the coils 2 and 3, for the reason that the field produces the coupling. Summing up, then, it will be seen that the closer the member 5 is to the two coupling coils, the smaller is the coupling between the coils 2 and 3, and, consequently, between the entire primary circuit and the whole secondary circuit.

The maximum effect of the tube or ring 5 is secured when the plane of the centre of the ring 5 coincides with the plane passing through the centre of the coils 2, 3; i. e., when the ring 5 is symmetrically placed in relation to the central plane of the coils 2 and 3. Movement of the ring from the central position reduces its eiTect, and thus increases the coupling between 2 and 3. The effect of the ring becomes negligibly small when it is entirely removed from the coils, with the adjacent edges of the ring and the coils slightly separated.

In order to define clearly the use of terms in this application, I wish it to be understood that when I refer to the members 5 being closer to the two coils, this signifies being closer to the central position at which maximum efiect is secured. I shall hereinafter refer to the coupling between the coils 2 and 3, when not subjected to the influence of the magnetic field set up between tube 5, as the original coupling magnetic field; and to the magnetic field set up around the tube 5, as the auxiliary, or additional, opposing magnetic field. By the term effective coupling field, it is to be understood that I refer to the field resulting from the effect of the auxiliary field on the original coupling field.

Fig. 2 illustrates the application of the basic arrangement in Fig. 1 to a single stage radio frequency amplifier of the band pass type. The input circuit of the tube 6, the latter being an electron dicharge device of the four electrode type, has impressed upon it the radio frequency energy to be amplified. The output circuit of the tube 6 includes coil i and coupling coil 2, the coils being shunted by a variable, tuning condenser I. The input circuit of a second electron discharge device 9, also of the four electrode type, includes an inductance coil 4 and a coupling coil 3, the two coils being shunted by a tuning condenser 8. The primary circuit I, 2, 1 receives electrical impulses from the device 6, and transfers them to the secondary circuit 3, 4, 8 in a manner well known to those skilled in the art. The voltage developed across the condenser I is impressed upon the input circuit of device 9.

As explained, in connection with Fig. 1, the 3 coupling between the primary and secondary circuits is controlled by the metallic member 5, whose position with respect to coils 2 and 3 can be varied. While the coils 2 and 3, and the tubular member 5 are drawn differently from the way they are represented in Fig. 1, it will be readily appreciated that this has been done only to make the circuit connections perfectly clear.

The structure and arrangement of the parts is the same as in Fig. l. The coils 2 and 3 are mounted rigidly, and the tubular member 5 is concentric with the coils, and supported on a sliding member which permits the member 5 to be disposed directly over the centre of the coils, or to be moved parallel to the common axis of the coils to a distance such that its effect is very small.

In the co-pending applications, referred to above, it was disclosed that constant band width" was secured by varying the coupling between the primary and secondary in such a manner that the co-efiicient of coupling was proportional to the wave length, or inversely proportional to the wave frequency. In the circuit in Fig. 2, constant band width is secured, therefore, by disposing the tubular member 5 at a distance from the coupling coils 2 and 3 at the lowest wave frequency to be received, and moving it closer to the coupling coils as the wave frequency increases. The movement of the tubular member may be effected by means of a cam, or

any well-known type of mechanical linkage connected to the common tuning control, shown in Fig. 2 in dotted lines. It is, of course, understood that the movement of the link may be effected by an independent control.

A modification of the basic arrangement of Fig. 1 is disclosed in Fig. 3. In the latter, the reference numerals have the same significance as in Fig. 1. The member 5, in this modification, is split at a point II. A condenser I 5 is connected in series with the ring at the point H where the ring is broken. In this modification, the split ring 5 is intended to be fixed in position with relation to the coils 2 and 3. The condenser ID has a low reactance at high frequencies and a high reactance at low fre quencies this being an inherent characteristic of a condenser.

Thus, the circulating current in the ring is reduced to a small value at low frequencies, but at high frequencies the circulating current is almost as great as it would be if the gap in the ring were closed. It will therefore be obvious that the reduction in coupling in this modification, is great at high frequencies and small at low frequencies. By suitably proportioning the windings 2 and 3, the ring 5, and the condenser Iii, the effective coupling field can be made to vary in such a manner as to maintain a constant band width characteristic over a range of frequencies.

An application of the modified circuit in Fig. 3 is disclosed in Fig. 4. The latter shows an antenna circuit In, II, I2, the coil I2 being coupled to the coil I3 in a manner similar to the arrangement of the coils 2 and 3 in Fig. 3. A split ring 5, having a condenser II" connected to it, is placed in the magnetic field of the coupling coils, as shown in Fig. 3. It will be understood that the coils I2 and I3 and the ring ,5 in Fig. 4 are intended to be similar to the members 2, 3, and 5 in Fig. 3, but are drawn differently to make the circuit connections perfectly clear. The coils I3, I4, I6 are tuned to the wave frequency by the condenser I5, and the coils I9, 20, are tuned to the wave frequency by the condenser I8. The tuned circuits I3, l4, I5, It (circuit A) and I8, I9, constitute a band pass tuning circuit and are coupled by the coupling coils I5 and 20 and the split ring 5 and condenser II".

The input terminals of the vacuum tube I! are connected to the tuned circuit I8, I9, 20 and in the output circuit is a tuned circuit C. The circuit C is coupled to the circuit B by a coupling arrangement similar tothat shown in Fig. 3, and another such coupling arrangement is used to couple the circuit B to the tuned circuit D including detector tube 2|. It will be recognized that the coupled tuned circuits C, B, and D constitute a band pass inter-tube tuning circuit.

Of course, the variable capacities may all be uni-controlled in any well-known fashion. It. will be readily appreciated from Fig. 4 that the efiective coupling fields between each pair of coupling coils will be varied simultaneously with the change in frequencies to be transferred from a primary to a secondary circuit, the reduction in coupling being great at high frequencies and small at low frequencies, the windings of each pair of coupling coils, each split ring, and each series condenser being so proportioned that a constant band width characteristic is maintained throughout the range of frequencies to be amplified.

In Fig. 4 I have shown two band pass tuning networks, one having two tuned circuits, and the other three tuned circuits. I have drawn the circuit in this way to make it clear that the coupling system shown in Fig. 3 (or that shown in Fig. 1, equally well) can be used in band pass networks having any number of tuned circuits.

The coupling between the antenna circuit Ill, II. I2 and the tuned circuit I3, I4, I5, I6 (circuit A) has no relation to the operation of the band pass net work, but represents an entirely different application of the principle disclosed in Figs. 1 and 3. It is well known in the art that it is desirable in certain cases to have the antenna so coupled to the radio frequency amplifier that the signal energy is transmitted more efficiently at low frequencies than at high frequencies.

This compensates for the greater amplification of the signal at high frequencies, and, by proper propcrtioning, it is possible to make the receiver equally sensitive at all wave frequencies. The coupling systems shown in Figs. 1 and 3 accomplish this result by coupling the circuit I3, I4, I 5, It (circuit A) more closely to the antenna circult III, II, I2 at low wave frequencies than at high wave frequencies.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications in the circuit arrangements, as well as in the apparatus employed, may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In combination, a pair of oscillation circuits, means for magnetically coupling said circuits, a fixed conductor independent of the coupling means disposed in proximity to said coupling means in such a manner that it produces a magnetic field in opposition to the original coupling field between the circuits, and capacitative means associated with said conductor of a magnitude such that the auxiliary opposing magnetic field varies simultaneously with a change in frequency of the current flowing through the oscillation circuits.

2. In combination, in a band pass amplifier, a pair of tuned circuits, interlinked coils connected to said tuned circuits for magnetically coupling the circuits, a metallic member independent of the coils, embracing said coils, for producing a magnetic field in opposition to said original coupling magnetic field, and additional means associated with said metallic member for varying the strength of said opposing magnetic field simultaneously with the change in frequency of the currents flowing in the tuned circuits.

3. In combination, in a band pass amplifier, a pair of tuned circuits, interlinked coils connected to said tuned circuits for magnetically coupling the circuits, a metallic ring independent of the coils, embracing said coils, for producing a magnetic field in opposition to said original coupling magnetic field, and additional means associated with said metallic ring for varying the strength of said opposing magnetic field simultaneously with the change in frequency of the currents flowing in the tuned circuits.

4. In combination, a pair of oscillation circuits, means for magnetically coupling said circuits, a split ring disposed in proximity to said coupling means in such a manner that it produces a magnetic field in opposition to the original coupling field between the circuits, and a capacity connected between the ends of said ring for varying the auxiliary opposing magnetic field simultaneously with a change in frequency of the current flowing through the oscillation circuits.

5. In combination, a pair of tunable oscillation circuits, means for magnetically coupling said circuits, a split ring disposed in proximity to said coupling means in such a manner that it produces a magnetic field in opposition to the original coupling field between the circuits, and a capacity in series between the ends of said ring for varying the auxiliary opposing magnetic field simultaneously with a change in frequency of the current flowing through the oscillation circuits.

6. A high frequency coupling system comprising an input circuit and an output circuit, each of said circuits including an inductance element, said inductance elements being placed together that there is a strong magnetic field therebetween When said inductances act alone, and means associated with said inductance elements for producing a magnetic field which is weaker than, and which opposes, said strong magnetic field, whereby the resultant magnetic field inter-linking said inductances is relatively weak as compared with said strong field, in which said means for producing the weaker magnetic field is a conducting ring surrounding 10 said coils.

RENE A. BRADEN. 

